1. Field of the Invention
The invention relates to an electric power steering system that assists an operation of a steering system of a vehicle by applying assist force to a rack shaft of the vehicle.
2. Discussion of Background
A conventional electric power steering system of this type is described in, for example, Japanese Patent Application Publication No. 2011-51387 (JP 2011-51387 A). FIG. 9 shows the structure of the electric power steering system described in JP 2011-51387 A. In a vehicle in which the electric power steering system is mounted, a rack shaft 90 in FIG. 9 makes a reciprocal motion in its axial direction (direction indicated by arrows a1, a2) in accordance with a driver's operation of a steering wheel and, as a result, the steered angle of steered wheels of the vehicle, that is, the travelling direction of the vehicle, is changed.
As shown in FIG. 9 as related art, the electric power steering system described in JP 2011-51387 A includes a cylindrical motor shaft 100 and a ball screw nut 110. The rack shaft 90 is passed through the motor shaft 100, and the motor shaft 100 is rotated about an axis m as a motor (not shown) is driven. The ball screw nut 110 is screwed to a thread groove 91 formed in the outer periphery of the rack shaft 90. An engaging portion 111 is provided at the right end portion of the ball screw nut 110. The engaging portion 111 is inserted into the motor shaft 100. An external thread portion formed on the outer periphery of the engaging portion 111 is screwed to an internal thread portion formed in the inner periphery of the motor shaft 100. With this configuration, the ball screw nut 110 is engaged with and coupled to the motor shaft 100. In the electric power steering system, the rotational motion of the motor shaft 100, which is generated as a the motor is driven, is converted to an axial linear motion of the rack shaft 90 via the ball screw nut 110. As a result, assist force is applied to the rack shaft 90.
In the thus structured electric power steering system, for example, if an excessive impact load acts on the rack shaft 90 when one of the steered wheels of the vehicle collides with a curb, the impact load is transmitted from the rack shaft 90 to the ball screw nut 110, and loosening may occur at a portion at which the engaging portion 111 of the ball screw nut 110 is fastened to the motor shaft 100. If it becomes impossible to transmit the driving force of the motor to the rack shaft 90 due to the loosening in the fastening structure, it may be not possible to apply assist force to the rack shaft 90.
Therefore, in the electric power steering system described in JP 2011-51387 A, a thin plate portion 121 of a flange member 120 that is fixedly fastened to an outer peripheral portion of the motor shaft 100 is folded and clinched to the flange portion 112 of the ball screw nut 110. In this way, the relative rotation of the ball screw nut 110 with respect to the motor shaft 100 is restricted. Thus, even if an impact load acts on the rack shaft 90, the connection between the motor shaft 100 and the ball screw nut 110 is maintained.
In order to manufacture the electric power steering system that has the above-described structure, a process of clinching the thin plate portion 121 of the flange member 120 to the flange portion 112 of the ball screw nut 110 is required. This leads to an increase in man-hours required to manufacture the electric power steering system, and consequently causes an increase in production cost.